1. Field of the Invention
The present invention relates to an image forming apparatus of electrophotography such as copiers, facsimile apparatuses and printers, and more particularly relates to a two-component development device in which a two-component developer born on the surface of a development roller is caused to rise in a form of a series of ears to contact a latent image bearing member and thereby a latent image on the latent image bearing member is visualized with toner of the developer.
2. Discussion of the Background
Two-component development devices using a two-component developer in which magnetic particles called carriers and toner are mixed are widely used in image forming apparatuses. Improvement in reliability and enhancement of image quality are demanded in image forming apparatuses, and in particular decreasing carrier adhesion is demanded for improving reliability and decreasing trailing edge omission is demanded for improving image quality.
In a two-component development device, a two-component developer including carriers and toner is born on the surface of a development roller, and in a development area where the development roller faces an electrostatic latent image bearing member, the developer born on the surface of the development roller is caused to rise in a form of a series of ears to contact the latent image bearing member and thereby a latent image on the latent image bearing member is developed with toner of the developer into a toner image. At this time, an electric force from the latent image bearing member and a magnetic force from the development roller are applied to carriers of the developer, and if the electric force from the latent image bearing member is greater than the magnetic force from the development roller, carriers that must remain on the development roller move together with toner and adhere to the latent image bearing member. This phenomenon is called carrier adhesion. When the toner image formed on the latent image bearing member is transferred onto a transfer sheet by a transfer device, the carriers adhered to the latent image bearing member are transferred to the transfer sheet together with the toner forming the toner image. This causes adverse effects to the transfer device. Further, when the toner image transferred onto the transfer sheet is fixed onto the sheet by a fixing device, the carriers transferred to the transfer sheet are also fixed to the sheet. This causes adverse effects to the fixing device. Thus, carrier adhesion is a factor decreasing reliability of an image forming apparatus.
To avoid carrier adhesion, it is conceivable to decrease the electric force applied to the carriers from the latent image bearing member by adjusting the charging potential of the latent image bearing member and the potential of the development roller, which, however, causes other problems in an image such as background soiling, etc.
A method of avoiding carrier adhesion is proposed in Japanese Patent Laid-open publication No. 8-15988, in which a magnetic flux density distribution curve in a development area is formed such that a width in a circumferential direction of a development sleeve of a development roller is narrower at the peak point side. By making the magnetic flux density in the development area locally larger, the action direction of a magnetic attraction force applied to carriers from the development roller is greatly changed. This results not only in that the magnetic attraction force from the development roller is made stronger than the electric force of a latent image bearing member, but also in that a rotation force acts on the carriers. Thereby, the attraction balance between the latent image bearing member and the carriers is rapidly lost, so that carrier adhesion is avoided.
A development roller in a two-component development device conveys a developer born on the surface of the development roller in a circumferential direction thereof, and the developer is caused to rise in a form of a series of ears at a part of the development roller coming close to a latent image bearing member. The ears of the developer have some widths in the circumferential direction of the development roller, and an electrostatic latent image on the latent image bearing member is developed with toner of the developer while the ears of the developer rub the latent image bearing member. At this time, if the widths of the ears of the developer are too large, a phenomenon that toner once moved to the latent image bearing member is scraped off the latent image bearing member occurs. This phenomenon is called trailing edge omission, and is a factor of deteriorating image quality. The trailing edge omission phenomenon is more noticeable as the widths of ears of the developer are wider.
The applicant of the present invention has proposed, for example, in JP Laid-open publications No. 2000-305360 and No. 2001-27849, a method of avoiding a trailing edge omission phenomenon by narrowing a half-value region width of a magnetic flux density distribution in a normal line direction of a development pole while making the magnetic flux density of the development pole relatively large. The half-value region width refers to an angular width at parts of a magnetic force distribution curve indicating values corresponding to one-half of the maximum (peak) normal line magnetic force. In the proposed method, however, because the attenuation ratio of magnetic flux density in the normal line direction of a development pole is relatively high (e.g., 40% or greater), depending upon the states of magnetic poles at the upstream and downstream sides relative to the development pole in the circumferential direction of a development sleeve of a development roller, a relatively large plunge is generated in the waveform of a magnetic force of the development pole and thereby carrier adhesion is occasionally caused.